National Water-Quality Assessment Program

Abstract

The U.S. Geological Survey has implemented the
National Water Quality Assessment program to
describe the quality of the surface- and ground-
water resources in 60 large areas or study units in
the Nation. The Georgia-Florida Coastal Plain
study unit was 1 of the first 20 selected for study
when the full-scale program was implemented in
1991. The study unit has an area of about 54,000
square miles and is located on the southeastern
coast of the United States. The primary source of
water supply in this study unit is water from the
Upper Floridan aquifer of the Floridan aquifer
system. The Upper Floridan aquifer is unconfined
or semiconfined in some parts of the study unit,
but in other parts is confined by the overlying
surficial aquifer system and other confining units.
The surficial aquifer system is also used for water
supply in some parts of the study unit. Three land
resource areas have been delineated in the study
unit on the basis of generalized soil categories:
the Central Florida Ridge, Coastal Flatwoods, and
Southern Coastal Plain. Predominant land use and
land cover, as classified in the 1970's, are forest,
agriculture, wetlands, and urban.

Nitrate data for water from the Upper Floridan
aquifer and the surficial aquifer system were
obtained from the National Water Information
System data base of the U.S. Geological Survey
for the years 1972-90. In the Upper Floridan
aquifer, the highest median nitrate (as nitrogen)
concentrations (0.43 and 0.26 milligrams per
liter) were in water samples from wells in
agricultural and urban areas where the aquifer
was unconfined or semiconfined. The maximum
contaminant level for nitrate (as nitrogen) in
drinking water of 10milligrams per liter was
exceeded in 25 of the 726 water samples from this
aquifer. These 25 samples were from wells in
urban areas. In water samples from the surficial
aquifer system, the highest median nitrate
concentration, 8.7 milligrams per liter, was for
water samples from agricultural areas in the
Central Florida Ridge. Nitrate (as nitrogen)
concentrations exceeded 10 milligrams per liter in
50 of the 421 water samples from wells
completed in the surficial aquifer system. Most of
these 50 water samples were from wells in
agricultural and urban areas (sewage spraying
areas) in the Central Florida Ridge.

Introduction

In 1991, the U.S. Geological Survey (USGS)
began implementation of the National Water
Quality Assessment (NAWQA) program. The
goals of this program are to describe the status
and trends in the quality of large, representative
parts of the surface- and ground-water resources
in large areas of the Nation and to provide a
scientific understanding of the natural and human
factors that affect the quality of these resources.
The NAWQA program is designed to integrate
water-quality information at different areal scales
and consists of 60 study units nationwide. The
study units have been selected to include parts of
most of the major river basins and aquifer systems
in the Nation. These study units cover areas of
1,200 to more than 50,000 square miles (mi2) and
represent about 60 to 70 percent of the Nationís
water use. The Georgia-Florida Coastal Plain
study unit was among the first 20 NAWQA study
units selected when the full-scale implementation
plan was initiated in 1991.

One of the first activities undertaken as part of the
NAWQA program was to compile, screen, and
analyze available water-quality data for nutrients
(nitrogen and phosphorus compounds) in the
study units. Nitrogen and phosphorus compounds
were selected because these compounds cause
eutrophication of surface waters, and because of
the possible health risk associated with the
nitrogen compound, nitrate, in drinking water.
This report provides a preliminary assessment of
nitrate concentrations in ground water in the
Georgia-Florida Coastal Plain study unit. The
assessment is based on data available from the
USGS water-quality data base, the National Water
Information System (NWIS). Nitrate
concentrations in ground water are the focus of
this report for several reasons: analytical data are
more abundant for nitrate than for other nitrogen
or phosphorus compounds, elevated
concentrations of nitrate in drinking water poses a
possible health risk, and nitrate is the only major
nutrient for which a maximum contaminant level
(MCL) has been established for drinking water.
The MCL for nitrate (as nitrogen) in drinking
water is 10 milligrams per liter (mg/L)
(U.S.Environmental Protection Agency, 1990).
This report briefly describes the relations between
nitrate concentrations and hydrogeology, land use,
and general soils groups as represented by land
resource areas. The report also lists the numbers
of samples that have concentrations of nitrate that
exceed the drinking-water MCL.

Description of the Study Unit

The Georgia-Florida Coastal Plain study unit, an
area of about 54,000 mi2, is located on the
southeastern coast of the United States (fig. 1).
The study unit has a large population of more than
7 million people that relies primarily on ground
water for drinking water. The primary source of
water in the study unit is the Floridan aquifer
system, which is one of the largest sources of
ground-water in the Nation. Surface-water
resources are also abundant in the study unit but
are not extensively used for drinking water
supply. Total freshwater use within the study unit
is approximately 4,300 million gallons per day
(Mgal/d), with 2,800 Mgal/d withdrawn from
ground water (R.M. Marella, U.S. Geological
Survey, written commun., 1992). Nearly
95 percent of the ground water used in the study
unit is withdrawn from the Upper Floridan
aquifer, the uppermost unit of the Floridan aquifer
system.

The Floridan aquifer system that underlies the
study unit is composed of a sequence of carbonate
(limestone and dolomite) rocks that include units
of high permeability (aquifers) as well as units of
low permeability (confining units). The rocks that
compose the aquifer system dip to the southeast
and are exposed at land surface in a thin band near
the northern boundary of the study unit and in
north-central Florida. The aquifer system thickens
in a southeasterly direction to a thickness of more
than 3,000 feet (ft) in the southern part of the
study unit. The Upper Floridan aquifer is
considered confined where the thickness of
overlying confining units is greater than 100 ft,
semiconfined where the overlying confining units
are less than 100 ft thick or are breached, and
unconfined where the units are virtually absent
and the Upper Floridan aquifer is at or near land
surface (Miller, 1986). These confining units
overlying the Upper Floridan aquifer contain the
intermediate aquifer system, also referred to as
the Brunswick aquifer in Georgia (fig.2). The
intermediate aquifer system consists of limestone,
clay, and sand, and is overlain by the surficial
aquifer system. Both of these aquifer systems are
used for water supply, but substantially less than
the Upper Floridan aquifer. The Claiborne aquifer
in Georgia is also used for water supply in
localized areas in northern parts of the study unity
where it underlies the Upper Floridan aquifer.

The general proportions of land use and land
cover in the study unit were determined using
data from the USGS geographic information
retrieval and analysis system (GIRAS) (Mitchell
and others, 1977). This land-use and land-cover
classification was done in the mid-1970's on a
national scale. The general level of land-use and
land-cover classification in the study unit includes
forest land, agricultural land, wetland, urban or
built-up land, water, rangeland, and barren land
(Anderson and others, 1976). The percentages of
area in the study unit covered by various land uses
and land-cover classification are listed in the
following table:

The study unit was subdivided into three land
resource areas based on generalized soil maps
prepared by the U.S. Department of Agriculture
in cooperation with the States of Georgia and
Florida (Perkins and Shaffer, 1977; Caldwell and
Johnson, 1982). These land resource areas are the
Coastal Flatwoods, Southern Coastal Plain, and
Central Florida Ridge (fig. 3). The Coastal
Flatwoods area includes the coastlines of Georgia
and Florida, and is characterized by nearly level
soils on plains and marshes and some low
terraces. The water table is generally near land
surface in the Coastal Flatwoods, and numerous
large springs flow along the Gulf Coast. The
Southern Coastal Plain is characterized by broad,
interstream areas with gentle to moderate slopes
and shallow to deeply incised valleys. The Central
Florida Ridge includes much of the central
uplands of Florida. This area is characterized by
hills, ridges, terraces, and many lakes, and is
marked by karst topography. Sinkhole lakes,
sinking streams, and springs are common in the
area. Most areas in the Central Florida Ridge have
few streams, with most of the drainage recharging
ground water.

Nitrate Data in National Water Information System

Nitrate data for ground-water samples and
ancillary information about the wells where the
samples were collected were retrieved from
NWIS for all wells that had available water-quality
and well-depth data. From October 1972
to September 1990, 1,380 wells in the study unit
were sampled at least once for analysis of
dissolved or total phases of nitrate. Because most
nitrate in ground water is in the dissolved phase,
no distinction was made between the total and
dissolved phases of nitrate in this preliminary
assessment. Dissolved and total nitrate as nitrogen
will be referred to as nitrate hereafter. Where
analyses for more than one sample from a well
were available, only the most recent analysis was
used in this preliminary assessment.

The nitrate data in NWIS represent ground-water
samples collected for many types of studies. The
objectives of the individual studies determined
the geographic and temporal extent of the data
collection, as well as the aquifer and depth zones
sampled. For example, regional ground-water
quality investigations usually include the
collection of water-quality data over much larger
geographic areas than do investigations of the
effects of landfills on local ground-water quality.
No effort was made to distinguish between the
data collected for resource appraisal and local
contamination assessment studies for this
preliminary assessment. The data retrieved
contained data for water samples from numerous
contamination studies; therefore, this summary
probably is biased toward areas of ground-water
contamination. A more rigorous assessment of the
NWIS data would be needed to identify and
eliminate some of the data bias.

Distribution of Nitrate in Ground Water

More than 90 percent of the samples for which
nitrate data are available were collected from
wells completed in either the Upper Floridan
aquifer or the surficial aquifer system
(fig. 4).
Only the nitrate concentrations in water from
these two aquifers are discussed here. For this
assessment, the variability of nitrate
concentrations in water from the Upper Floridan
aquifer was determined for various categories of
degree of confinement and land use near the
sampled well. Nitrate distribution in water from
the surficial aquifer system was determined for
the three land resource areas and categories of
land use.

Land use at each well site was determined from
the USGS GIRAS data (Mitchell and others,
1977) and updated as necessary. The major land-
use classifications were grouped into categories
that were used to assess the relation between
nitrate concentrations and land use. These
categories were urban, agricultural/rangeland,
forest, and barren. Agricultural areas and
rangeland were combined into one category
(agricultural/rangeland), because only a small
number of ground-water samples were collected
in rangeland areas. Barren areas were included
only for sampling from the surficial aquifer
system, because less than 5 percent of the water
samples from the Upper Floridan aquifer were
collected from wells in barren areas. Most of the
ground-water nitrate data stored in NWIS were
collected in Florida; many counties in Georgia
had no ground-water nitrate data
(fig. 4).

Of the 726 water samples collected from wells
completed in the Upper Floridan aquifer in the
land-use categories of agricultural/rangeland,
forest, and urban, 137 samples (19 percent) had
nitrate concentrations less than analytical
detection limits. These detection limits ranged
from 0.01 to 0.30 mg/L during 1972-90 because
of changes in analytical methods. Detection limits
of 0.01 or 0.02 mg/L were most common (125 of
the 137). For this preliminary assessment, all
nitrate concentrations less than analytical
detection limits were set equal to the detection
limits established at the time of analysis.

The variability of nitrate concentrations in water
from the Upper Floridan aquifer is affected by the
presence of and thickness of the overlying
confining units and by land use near the well site
(fig. 5). Nitrate concentrations in water samples
from the Upper Floridan aquifer generally were
low (less than 0.50 mg/L). Median concentrations
in the categories ranged from 0.02 to 0.43 mg/L.
Water from the confined areas of the Upper
Floridan aquifer had the lowest median nitrate
concentrations in all three land-use categories,
with median concentrations less than or near
analytical detection limits. Water from unconfined
areas of the aquifer had the highest median nitrate
concentration in all three land-use categories.
Median nitrate concentrations in water from
semiconfined areas of the aquifer in each land-use
category was between the medians for confined
and unconfined areas. The two highest median
nitrate concentrations, 0.43 and 0.26 mg/L, were
in water from wells completed in unconfined
areas of the aquifer in agricultural/rangeland and
urban land-use areas, respectively
(fig. 5). In
forest areas, the median nitrate concentration in
water from unconfined areas of the aquifer was
higher than that in semiconfined and confined
areas but was lower than the median
concentrations in unconfined areas of the aquifer
in the other land-use areas.

The drinking water MCL of 10 mg/L for nitrate
was exceeded in 25 of the 726 water samples (6
percent) from the Upper Floridan aquifer. All 25
water samples were from wells located in areas
where the aquifer is unconfined or semiconfined,
and 24 of these samples were collected from wells
in urban areas near sewage treatment plant
effluent-spray areas or landfill sites (Berndt,
1990; 1993, and Trommer, 1992). One water
sample in which the MCL for nitrate was
exceeded was from a well at a dairy farm in an
agricultural/rangeland area (Andrews, 1992).

Of the 421 water samples from wells completed
in the surficial aquifer system in the land-use
categories of agricultural/rangeland, barren,
forest, and urban, 60 (14 percent) had nitrate
concentrations less than analytical detection
limits. Detection limits ranged from 0.008 to 0.10
mg/L. For this preliminary assessment, nitrate
concentrations less than analytical detection
limits were set equal to the detection limit at the
time of analyses, except for the 12 samples with a
detection limit of 0.008 mg/L. For those samples,
the detection limit was rounded to 0.01 mg/L.
These 12 samples were from wells in forest areas
in the Central Florida Ridge.

The median nitrate concentrations in water from
the surficial aquifer system differed among
various categories of land use near the well site
and, to a lesser extent, by land resource area
(fig.6). No nitrate data for water from the
surficial aquifer system were available in the
Southern Coastal Plain land resource area for
agricultural/rangeland, barren, and forest areas.
For the surficial aquifer system, the highest
median nitrate concentration, 8.7 mg/L, was for
water samples from wells in agricultural/
rangeland areas in the Central Florida Ridge. In
contrast, the median nitrate concentration for
water samples from wells in agricultural/
rangeland areas in the Coastal Flatwoods was
0.02 mg/L. In the four categories for barren and
forest areas in the Central Florida Ridge and
Coastal Flatwoods, median nitrate concentrations
in water from the surficial aquifer system were
less than or near detection limits. In the Central
Florida Ridge and Coastal Flatwoods, the median
nitrate concentrations in water from the surficial
aquifer system in urban areas were 0.16 and 0.05
mg/L, respectively. The median nitrate
concentration in samples from urban areas in the
Southern Coastal Plain was 0.50 mg/L.

Concentrations of nitrate exceeded the MCL for
drinking water in 50 of the 421 water samples (12
percent) from wells completed in the surficial
aquifer system. Nitrate concentrations that
exceeded the MCL for nitrate were present in 25
of the 50 samples from wells located in the
Central Florida Ridge and in agricultural/
rangeland areas. Many of these 25 wells were in
citrus agricultural areas in the Central Florida
Ridge (E.R. German, U.S. Geological Survey,
written commun., 1991; and Lee and others,
1991). In urban areas in the Coastal Flatwoods
land resource area, 18 water samples from wells
completed in the surficial aquifer system had
nitrate concentrations that exceeded the MCL for
nitrate. Many of these samples were from wells
located at a site of sewage-effluent disposal
(Trommer, 1992). Four samples from wells
located within agricultural areas in the Coastal
Flatwoods had nitrate concentrations that
exceeded the MCL for nitrate. Three other
samples with nitrate concentrations that exceeded
the MCL were from wells in forest and urban
areas in the Central Florida Ridge and from an
urban area in the Southern Coastal Plain.

This preliminary assessment of available nitrate
data for ground water in the Georgia-Florida
Coastal Plain study unit indicates a possible
relation between land use and nitrate
concentrations. Further study is needed to
determine the relation between land uses and
ground-water quality. Studies might consist of
data-collection activities, such as random
sampling of ground-water within selected land-
use areas of interest in the Georgia-Florida
Coastal Plain study unit. Analysis of these
ground-water samples for a comprehensive suite
of chemical constituents and physical properties
would further aid in determining the relation
between land use and ground-water quality.

References

Anderson, J.R., Hardy, E.E., Roach, J.T., and
Witmer, R.E., 1976,

A land use and land cover
classification system for use with remote sensor
data: U.S. Geological Survey Professional Paper
964, 28p.

Andrews, W.A., 1992,

Reconnaissance of water
quality at nine dairy farms in north Florida: U.S.
Geological Survey Water-Resources
Investigations Report 92-4058, 39 p.

Berndt, M.P., 1990,

Sources and distribution of
nitrate in ground water at a farmed field irrigated
with sewage treatment-plant effluent, Tallahassee,
Florida: U.S. Geological Survey Water Resources
Investigations Report 90-4006, 33 p.